PROJECT SUMMARY This proposal is a competitive renewal for a unique study that measures the volume, function, and development of striatal-cerebellar circuity in children at risk for Huntington's Disease (HD). The standard assumption is that HD is a degenerative disease of the striatum. However, research supports supported the notion that a crucial component of the pathoetiology of HD is abnormal brain development. The grant was originally funded in 2009 and dubbed the Kids-HD program, designed to investigate this hypothesis by the study of children at risk for HD (those with a parent or grandparent with HD). The at-risk participants are genotyped and those who are gene-expanded (GE) are compared to those who are gene non-expanded (GNE). Gene knock-down therapy ? Antisense Oligonucleotides or ASOs ? are currently entering Phase III studies and hold promise for treatment of patients in early stages of disease (by preventing further decline). If ASOs fulfill that promise, the next step will be preventive therapy ? giving the ASO early enough (potentially to children) to prevent symptoms from occurring. The growth and development of the striatum is vital to understand as this is the primary site of disease pathology. Yet, knocking down a gene that is vital to development of these structures must be approached with an abundance of caution. Human brain development is prolonged, with striatal maturational changes occurring up through 30 years of age. Therefore, discriminating ongoing development/maturation with the degenerative phase of the disease may be key in knowing when to administer and ASO. Our preliminary data suggest that a novel blood biomarker ? Neurofilament light (NfL) rises within roughly 20 years of onset but is normal prior to that, suggesting it is not present in development, but is seen at the very beginning phases of degeneration. Rationale for renewal and expansion (5 sites across the US) include: 1) increase sample size for replication of original findings with sufficient power to detect CAG-specific effects and 2) model the entire period of brain development (up to age 30 rather than only up to age 18); 3) evaluate the utility of a blood biomarker of neural dysfunction, Neurofilament light (NFl) that may help delineate the earliest phases of degeneration.